Check the log files for entries.
Did I understood that correct and the ftp login is just not working? Did you manually created that login or is that a system generated login?
Since I am not quite sure what exactly you mean with backup hosting, I will assume for now its a self generated account or via cmd not not through the webmin/virtualmin gui itself.
Did you check to enable unix auth for the FTP? Which one do you use? Can you be a bit more specific than just basic information?
I’m trying to backup using: Backup Virtual Servers under Virtualmin and I have an external backup storage. I’m not sure what is not understandable here.